Hard to tell since there are not too many Ninja's on the used market. If it has been well maintained, seems like a fair price. Overall the N1K seems pretty bullet proof. I haven't really seen anything about common issues.
The vibration is really only present above 6k rpm, so unless you're cruising at high speed for extended periods of time, not a huge issue. 5.5krpm is about 80mph on my '14. I think it is slightly higher on the '11 but the difference is negligible.
Engine braking; yes, there is a good amount. Supposedly Ivan's ECU reflash will fix that. I haven't done it yet but most likely will.